CLICKHOUSE_CLUSTER_EXTERNAL_ENGINE_UNSUPPORTED

External replication requires direct, independent, non-replicated MergeTree-family targets. A Replicated*MergeTree, Distributed, Shared, or otherwise unsupported engine cannot use per-member external staging safely.

Use external mode only for the admitted non-replicated layout. Use internal mode for a compatible Replicated*MergeTree topology whose complete inventory reports internal_replication=true.

No engine is substituted and no local publication fallback is attempted. See the mode matrix.